Further battles with the green giant: a comparison of factors critical to reed canarygrass control in herbaceous vs. forested wetlands

Background/Question/Methods

Phalaris arundinacea (reed canarygrass) is an aggressive invader of many wetland types in temperate North America. Its wide distribution and unusual ecological amplitude make it difficult to control, but provide a unique opportunity to compare the factors influencing the invasion of a single species across multiple habitat types. A conceptual model was recently published which summarizes knowledge of the factors influencing the invasion and restoration of Midwestern sedge meadows impacted by Phalaris. We have adapted that model for the floodplain forest of the Upper Mississippi River, using recent experiments to evaluate factors that promote the persistence of various ecosystem states, or facilitate transitions between them.

Results/Conclusions

Similar to herbaceous wetlands, competition and litter are critical to the maintenance of Phalaris monocultures in the UMR floodplain, inhibiting the recovery of native vegetation. Hydrology also exerts a strong influence in both ecosystems. In sedge meadows, high water levels promote Phalaris; in the UMR, flood-related mortality among tree seedlings in a restoration site allowed Phalaris to re-invade. In contrast to the sedge meadow model, in which herbivory was not included as a major influence on restoration success, browsing by Odocoileus virginianus (white-tailed deer) strongly influences tree seedling height growth in the UMR floodplain. The shorter stature of browsed seedlings makes them vulnerable to repeated browsing and inundation during flood events, and prevents them from shading Phalaris. Furthermore, Phalaris appears to provide habitat for Microtus pennsylvanicus (meadow voles), which can girdle young trees. High herbivore densities in restorations could thus promote a back-transition to the invaded state, or slow stand development in restored sites. A final contrast between herbaceous and forested wetlands invaded by Phalaris is that while restored sedge meadows are regarded as differing significantly from relict sites, early results suggest that restored sites in the UMR floodplain will mature into forest stands that are similar to natural ones in terms of structure and diversity.